Liquid crystals (LC) are widely used for electronic displays. In these display systems, an LC layer is typically situated between a polarizer layer and an analyzer layer. The analyzer is oriented such that its absorbing axis is perpendicular to that of the polarizer. Incident light polarized by the polarizer passes through a liquid crystal cell is affected by the molecular orientation in the liquid crystal, which can be altered by the application of a voltage across the cell. By employing this principle, the transmission of light from an external source, including ambient light, can be controlled. The energy required to achieve this control is generally much less than that required for the luminescent materials used in other display types such as cathode ray tubes. Accordingly, LC technology is used for a number of electronic imaging devices, including but not limited to digital watches, calculators, portable computers, electronic games for which light weight, low power consumption and long operating life are important features.
Contrast, color reproduction, and stable gray scale intensities are important quality attributes for electronic displays, which employ liquid crystal technology. The primary factor limiting the contrast of a liquid crystal display is the propensity for light to “leak” through liquid crystal elements or cell, which are in the dark or “black” pixel state. Furthermore, the leakage and hence contrast of a liquid crystal display are also dependent on the angle from which the display screen is viewed. Typically the optimum contrast is observed only within a narrow viewing angle centered about the normal incidence to the display and falls off rapidly as the viewing angle is increased. In color displays, the leakage problem not only degrades the contrast but also causes color or hue shifts with an associated degradation of color reproduction.
Current rapid expansion in the liquid crystal display applications in various areas of information display is largely due to improvements of qualities. One of the major factors measuring the quality of such displays is the viewing angle characteristic (VAC), which describes a change in a contrast ratio from different viewing angles. It is desirable to be able to see the same image from a wide variation in viewing angles and this ability has been a shortcoming with liquid crystal display. Also for a potential application of liquid crystal display for moving pictures, we need to have a display mode with a high-speed response.
A bend aligned nematic liquid crystal cell, also referred as an Optically Compensated Bend (OCB) cell 50, is a nematic liquid crystal cell based on the symmetric bend state. In its actual operation, the brightness of the display using the bend aligned nematic liquid crystal cell 50 is controlled by an applied voltage or field that leads to a different degree in the bend orientation as shown in FIG. 1A and FIG. 1B. It has advantages in VAC and response speed over conventional displays, such as Twisted Nematic mode. The fast response is the results of switching between the different bend states; changing from one bend to another does not cause a reverse torque that prevents a fast rotation of liquid crystal molecules at the middle of the cell. The better VAC with a proper compensation is due to the symmetric molecular arrangement inside of the liquid crystal cell. In FIG. 1A and FIG. 1B, the liquid crystals 12 are sandwiched between two substrates 10. In the X-Z plane of the XYZ coordinate system 22, the liquid crystals 12 take the bend structure, which is symmetric around the cell mid plane 20. This bend structure is invariant in the Y direction. The ray 16 coming from left to right is close to perpendicular to the molecules at the bottom part 18 of the cell experiencing larger birefringence. In the upper part 24 of the cell, on the other hand, the ray 16 is nearly parallel to the molecules giving lower birefringence. The opposite happens to the ray 14 (traveling from right to left); lower (higher) birefringence in the bottom (upper) region of the cell. Therefore, the rays 14 and 16 experience the similar optical path. In another words, the OCB cell 50 has left-right symmetry. Since the OCB cell 50 operates entirely in the bend states, this symmetry holds with or without an applied field as indicated in FIGS. 1A and 1B. This fact indicates an intrinsically widened VAC, which is a stark difference from the conventional Twist Nematic mode. The Twisted Nematic mode does not maintain the aforementioned left-right symmetry.
As a reflective type OCB mode, one can use a Hybrid Aligned Nematic (HAN) liquid crystal cell 51 as shown in FIG. 1C. The HAN cell 51 has different boundary conditions such that the liquid crystals are vertically aligned at the bottom part 19 of the cell while they are tilted at the upper part 25 of the cell. This is really a half of the bend aligned nematic liquid crystal cell with a reflective plate 13 on one end. The principle of the operation of the HAN cell 51 is the same as that of the OCB 50 except that the light ray is reflected by the plate 13. The incoming ray 17A is nearly parallel to the liquid crystal at the top part 25 of the HAN cell 51, therefore it experiences a smaller birefringence. However, the reflected ray 17B is almost perpendicular to the liquid crystal at the top part 25 of the HAN cell 51 and sees a larger birefringence. Thus the HAN cell is operated in the same way as the OCB cell.
A practical application of bend aligned nematic liquid crystal cells, however, needs optical compensating means to optimize the VAC. Bend aligned nematic liquid crystal cells, similar to other modes, comprise of liquid crystal materials with an optical anisotropy and polarizers. Thus, the VAC suffers deterioration in contrast when viewed from oblique angles. Also, the bend state is not stable when a tilt of liquid crystal molecules is low at the cell substrates 10. Therefore, in order to maintain the bend orientation in the cell, one has to enforce a high tilt angle at the cell substrates 10. This leads to a high average refractive index in the direction normal to the cell surface plane (along the Z axis) and small one in the XY plane. Thus as a compensating film, one with the optic axis (the direction in which light does not see birefingence) lying in the film normal with negative out of plane birefringence (a negative C-plate) is effective to some extent. Other aspect of the compensation comes from the fact that the bend structure is contained in the XZ plane in FIGS. 1A and 1B. Unless an applied field is high enough to make the liquid crystals sufficiently perpendicular to the substrates 10, there appears a phase retardation in the XY plane. This in-plane phase retardation makes a C-plate compensator impossible to shield the light resulting in an unsatisfactory contrast ratio.
Uchida (Japanese Patent 07084254 A2, U.S. Pat. No. 6,108,058) and Bos (U.S. Pat. No. 5,410,422) used a biaxial plate and a negative C-plate to compensate the bend aligned nematic liquid crystal cell in a black state, respectively. FIG. 2 shows a prior art liquid crystal display 98 comprising a bend aligned nematic liquid crystal cell 50, a biaxial plate 34, and polarizers 32, 42. The biaxial plate 34, used to compensate the liquid crystal cell, had indices of refraction satisfying ny>nx>nz represented by an ellipsoid of index 36. It is placed between the bend aligned nematic liquid crystal cell 50 with a voltage source 38 and the top polarizer 32. Polarizers 32, 42 are crossed. The out of plane component of the phase retardation of the biaxial plate 34, {nz−(nx+ny)/2}d, where d is a thickness of the biaxial plate 34, is negative and compensates the positive contribution from the cell. The in-plane component (ny−nx)d would ensure a sufficiently dark state at the normal viewing angle with a finite applied voltage. This method improved the VAC of bend aligned nematic liquid crystal cells, yet the results remained unsatisfactory. The orientation of liquid crystal varies continuously along the Z-axis giving a change in the refractive index in the cell thickness direction. On the other hand, the index of refraction does not undergo any changes across the thickness in the above compensation films including both the biaxial plates and the negative C-plates.
Discotic liquid crystals consist of disk-like mesogenic molecules that are typically optically negative uniaxial materials. Uniaxial negative materials have three indices of refraction satisfying n3<n1=n2, where n3 is a refraction index in the direction of the optic axis. Utilizing these materials, Mazaki et al. (U.S. Pat. No. 6,124,913) and Mori et al. (U.S. Pat. No. 5,805,253) independently pursued the idea of compensating the bend aligned nematic liquid crystal cell. The compensation film was made from the discotic materials in which the direction of molecules varies in the thickness direction. The discotic film provides both in-plane phase retardation as well as effective out of plane negative retardation. By adjusting the discotic molecules direction inside of the film along with other parameters, they obtained widened VAC with bend aligned nematic liquid crystal cells.
While the above-mentioned methods have improved the viewing quality of the bend aligned nematic liquid crystal displays, the overall VAC remains poorer than it is desirable. It is a problem to be solved to provide a compensation film for a bend aligned nematic liquid crystal cell that improves the viewing angle characteristic of the display.